docs: def clean_times(feed: "Feed") -> "Feed":
      """
      In the given "Feed", convert H:MM:SS time strings to HH:MM:SS time
      strings to make sorting by time work as expected.
      Return the resulting "Feed".
      """
  
      def reformat(t):
          if pd.isnull(t):
              return t
          t = t.strip()
          if len(t) == 7:
              t = "0" + t
          return t
  
      feed = feed.copy()
      tables_and_columns = [
          ("stop_times", ["arrival_time", "departure_time"]),
          ("frequencies", ["start_time", "end_time"]),
      ]
      for table, columns in tables_and_columns:
          f = getattr(feed, table)
          if f is not None:
              f[columns] = f[columns].applymap(reformat)
          setattr(feed, table, f)
  
      return feed